1. Technical Field
The present invention relates generally to an improved data recording system, and in particular, but not exclusively, to a system and method for mitigating the effects of inoperable servo readers used for data track positioning on tape drives.
2. Description of Related Art
Tape drives are peripheral mass storage devices often used to archive data on tapes for later access. Tape drives are also used as random access devices in data storage applications where the cost of storage might be important and access time is of less significance. For example, many banks and financial institutions, insurance companies, and government agencies may use tape drives with applications regarding clients, accounts, patients, and other items of interest. In other applications, huge amounts of data are stored directly on tape for later retrieval and analysis.
In accessing data on a tape, a read or write head in a tape drive is positioned over one or more tracks on the tape. Typically, two or more servo tracks on the tape are used for aligning the read or write head to the tracks containing the data. For example, servo control information can be located in the servo tracks. This servo control information can be a repeating pattern of magnetic flux transitions. Alternatively, optical servo tracks may be found on the tape. Servo readers read the servo control information from the servo tracks on the tape, and generate position signals used to control the positions of associated data read and write heads over tracks containing the data.
If a read or write head drifts from an optimal position relative to a track on the tape, a servo reader generates an error signal. The error signal increases as the read or write head drifts further from the optimal location relative to the data on the track. If the read or write head drifts too far relative to the data on the track, errors and data recovery problems occur in accessing the tape. Existing systems adjust the position of the read or write head, or the position of the tape relative to the read or write head, to minimize the error signal generated by the servo reader during access of the tape.
Evidence indicates that, over time, tape dimensions are distorted over the length of the tape. In addition, such tape distortion can worsen with a combination of time, temperature, and use. As such, significant problems result from tape distortion. For example, the data tracks on a tape can shift position relative to the data read and write heads, resulting in the loss or overwriting of data. Also, distorted servo tracks can degrade the ability of the tape servo system to acquire and follow the servo tracks, resulting in degraded performance. Notably, these tape distortion problems are exacerbated if a servo reader becomes inoperable. The temporary or long term loss of a servo reader compromises the quality of the position error information received by the tape servo system, which in turn, compromises the effectiveness of the servo system's track alignment or track following ability. Ultimately, the combined problem of tape distortion and compromised track alignment or track following ability significantly increases the probability of data loss.
For example, assume (for illustrative purposes) that a conventional read/write head assembly includes two operable servo readers. The head assembly may contain a columnar arrangement of elements including, for example, an upper servo reader, a lower servo reader, and a plurality of data read/write elements located between the servo readers. With two servo readers, the track following operation for a distorted tape is typically a compromise. One servo reader is positioned “high” relative to a servo pattern being tracked, and the other servo reader is positioned “low”. As a compromise, the two servo readers “split the difference” for the track following error, in order to minimize the tracking offsets for the data read and write elements involved.
If one of the servo readers becomes inoperable, the servo system cannot position the read/write head assembly in an optimal, compromise location. Consequently, the data read and write elements located near the operating servo reader typically follow their respective data tracks closely. However, the read/write data elements located farther away from the operating servo reader typically have tracking offsets that substantially reflect the distortion of the tape.
For example, each servo reader tracks a respective servo stripe on the tape. Using two servo readers, if the amount of tape distortion between the two servo stripes is expressed as a distance value, N, then the optimal position of the read/write head assembly is with one servo reader located at +N/2 and the other servo reader located at −N/2. In this case, the data read/write elements located midway between the servo readers track very well with little or no offset. However, the data read/write elements nearest the servo readers can be misaligned with an offset as large as N/2. Under these conditions, and now assuming that the lower servo reader has failed, the upper servo reader can still track its servo pattern very well with little or no offset. Also, the data read/write elements located nearest the upper servo reader can track very well. However, the data read/write elements located nearest the lower, inoperable servo reader are misaligned by a factor of N. As such, if one of two servo readers fails, and a distorted tape is also involved, then the data read/write elements can be misaligned with an offset difference factor as high as 2.
Therefore, it would be desirable to have a method and system for mitigating the effects of inoperable servo readers used for data track positioning on tape drives.